This invention relates to a method of coating travelling web material by the curtain coating method.
In coating apparatus of the curtain coating type a travelling web is coated by causing a free-falling vertical curtain of coating liquid to impinge on to the travelling web to form a layer thereon. An apparatus of this type is described in British patent specification No. 1,276,144. In this specification a single layer of coating liquid is formed on the travelling web. However, in British patent specification No. 1,276,481 a similar apparatus is described wherein a plurality of layers of coating liquid are formed simultaneously on the travelling web.
In curtain coating methods the width of the falling curtain is maintained by the provision of edge guides which define the boundaries of the curtain. The presence of these guides, however, causes non-uniformities of the coating when the whole of the curtain is applied to the moving web. By making the curtain wider than the web to be coated by an amount at least equal to this non-uniform region at each edge, the coating applied to the object or web becomes substantially uniform. When coating a single layer on a travelling web the liquid at the ends of the curtain which overflows at the edges of the travelling web may be recirculated into the coating liquid supply. However, when a plurality of layers are coated simultaneously the liquid overflows from the edges of the web which is derived from the component layers becomes mixed and thus cannot be recirculated. Usually the liquid used to form coatings on webs is costly and thus it is desirable to use all the liquid present in the curtain and not to waste any of it.